People have little marker things called antigens on the surface of their red blood cells. The two most important ones are ABO system antigens and the Rh system. The BabyMed interactive blood group calculator calculates possible blood groups for the ABO system:
The ABO Blood Group System
The Rhesus (Rh) system
Besides A and B there is another important antigen on red blood cells, the Rh antigen which is also known as the D-Antigen.
The number of people who are Rh-positive varies by population:
In white Europeans about 16% of the population will have Rh negative blood types (and 8% will be Rh-positive). In the other populations of the world, the frequency of Rh negative types will be much lower; in Africans, only 9 people in 10,000 will be Rh negative, and in the non-African, non-European portion of the world, only 1 person in 10,000 will be Rh negative.
In the United States, about 15% of the general population is Rh negative; almost 20% of European Americans are Rh negative (because many Americans have western European ancestry), and approximately 5-10% of African Americans are Rh negative (because of the flow of European genes into the African American population). Fewer than 1% of Asian and Native Americans are Rh negative. More HERE.